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Abstract

Located at the southern section of the Artic Mid-Ocean Ridge, the Jan Mayen Vent Fields (JMVF)
consist of three main hydrothermal sites, the Troll Wall, Perle & Bruse and Soria Moria. These
sites contain numerous Fe-deposits, located distal to high-temperature venting sites. A recent
study of such Fe-deposits from the Troll Wall reports of abundant neutrophilic Fe-oxidizing
bacteria (FeOB), at locations with active low-temperature venting. The same study suggests that
the stratified structure, and textural and chemical variations of the laminae and layers of the
deposits reflect changes in physicochemical conditions (i.e. temperature, fluid dynamics, pH,
nutrient availability), which govern the formation processes, such as habitability and growth of
biomineralizing FeOB, and abiotic mineralization. In this study samples of Fe-depositsfrom both
the Troll Wall and Perle & Bruse vent fields are characterized by scanning electron microscopy
(SEM) and geochemical analysis, with the aim of establishing possible intra- and inter-field
textural and geochemical variations at these sites, and if such variations can be explained by
similar physicochemical changes.
The textural results reveal that the samples from both fields are built up of stacked sequences
of five distinct laminae or laminated layers of different colour, thickness, porosity and
microtextures, separated by internal cavities. A yellow and a light brown layer with a highly
porous framework of 10-50 μm thick, 200μm to >1000μm long bundles of 0.3-1μm wide fibres,
which apparently grew inward from an outer glass-like lamina and into the cavities, form the
innermost layers in each sequence. The fibres were likely formed through nucleation onto
extracellular polymers (EPS). Secondary mineral coating and attached twisted FeOB stalks on
the bundles in the light brown layer suggest a further development from the yellow stage
through different biotically and abiotically dominated stages. Clusters of 2-3 μm wide,
branching tubes (Y-guys) associated with biomineralizing FeOB are mainly converging around
the glass-like lamina. Massive, 5-15 μm wide, and 50-300 μm long unidirectional filamentous
structures comprising most of the outermost brown and dark brown layer, are likely formed
from connecting nodular, abiotic precipitates, indicating periods of poor biotic growth
conditions. Lower amount of secondary mineral coating, stronger branching of Y-guys, and
greater general abundance of other likely biosignatures of microaerophilic FeOB in the Perle &
Bruse sample suggest better growth conditions at this site. The geochemical data confirm that
the samples from both vent fields have a similar composition, with approximately 50-80 wt%
Fe2O3 and 20-35 wt% SiO2, along with low contents of base metals. Mn is enriched in the surface
layer relative to the interior, although the concentration is markedly lower in the Perle & Bruse
sample compared to the Troll Wall sample. The enrichment likely reflects Mn-oxyhydroxides
precipitation at the oxic surface during periods of hydrothermal quiescence. Thus, the variation
in Mn-content between laminae at both sites suggests variations in hydrothermal input
throughout the formation of the deposits at both sites, with generally a more intermittent
hydrothermal activity at the Troll Wall site than the Perle & Bruse site. Chondrite-normalised
REE patterns for the Troll Wall deposit are similar to that of seawater and low-temperature
hydrothermal fluid previously reported for this field. However, REE patterns with similarity to
Jan Mayen basalts for the Perle & Bruse deposit suggest a higher fluid temperature resulting in
stronger leaching of the subsurface rocks at this site. Negative Eu anomalies indicate that the
hydrothermal fluid at both sites never reached temperatures above 200-250°C.
This study demonstrates that hydrothermal fluctuations and associated shifts in the position of
the redox-gradient and nutrient availability likely explain the formation of the laminated
structure and the activity of Fe-oxidizing bacteria in marine hydrothermal Fe-deposits. The
study also demonstrates that differences in intra-and inter- field textural and chemical
composition do exist between the investigated Fe-deposits, and that these differences are likely
caused by variation in the venting styles and physicochemical conditions of the hydrothermal
fluids. To this end, conceptual models for the formation of the Perle & Bruse and Troll Wall Fedeposits
and associated microtextures are proposed.